Systems for electrostatic image reproduction are known in the art. These systems include apparatus for creating a latent electrostatic image on an image forming surface, such as a photoreceptor, through the definition of image and background portions on the photoreceptor surface at different electrical potentials, apparatus for developing the latent image including contacting the latent image with a toner including charged toner particles and apparatus for transferring the developed electrostatic image to a final substrate. This transfer may include the step of first transferring the developed image to an intermediate transfer member for subsequent transfer to the final substrate.
In general, transfer of the developed image from the photoreceptor is aided by an electric field which is generated by the electrical potential difference between a substrate (which can be the final substrate or an intermediate transfer member if one is present) and the image portions on the photoreceptor underlying the developed image. In order to assure good transfer the electric field must be maintained within a given range. In so-called direct copiers (or in "write-white" printers), projections of the image areas of the original (i.e., those areas which are black) on a photoreceptor do not discharge corresponding image portions of the photoreceptor. Projections of the background areas, which are lighter, discharge the voltage on corresponding background portions of the photoreceptor. The potential difference between the background portions (which are near zero volts) and the image portions are of the order of 500 to 1000 volts. In order to assure good transfer, the potential generally required on the substrate is substantially greater than this potential difference, causing electrical discharge between the background portions and the substrate.
It is known for this direct imaging case to irradiate the photoconductor, before transfer of the image therefrom, with strong light which penetrates through the developed image and discharges the charged regions underlying the developed image. The electrical potential on the paper or intermediate transfer member can then be greatly reduced, avoiding or greatly reducing discharge and damage to the photoreceptor and/or the surface of the intermediate transfer member. Examples of this process are shown in U.S. Pat. Nos. 3,784,300, 4,039,257 and 4,853,736 the disclosures of which are incorporated herein by reference.